Achieving low defect densities throughout a semiconductor active device layer is important for the fabrication of a commercially practical nitride-based semiconductor device. As described in U.S. patent application Ser. No. 11/503,660 (“the '660 application”), the entire disclosure of which is hereby incorporated by reference, it is possible to form large-diameter, low-defect-density AlN substrates. However, many desirable device applications preferably incorporate device layers based on alloys of AlN, GaN, and InN to be grown on the AlN substrate. As the concentration of GaN and InN is increased, the lattice mismatch with respect to the AlN substrate also increases. For instance, the lattice parameter in the c-plane of GaN is approximately 2.4% larger than that of AlN. When a lattice-mismatched layer is epitaxially grown on a substrate, the initial layer typically grows pseudomorphically—that is, the epitaxial layer will be compressed (experience compressive strain) in the plane of the substrate surface if the intrinsic lattice parameter of the substrate is smaller than that of the epitaxial layer. The epitaxial layer will be stretched or put under tensile strain when the intrinsic lattice parameter of the epitaxial layer is smaller than that of the substrate. However, as the thickness of the epitaxial layer is increased, the strain energy in the epitaxial layer will grow and, typically, the layer will find some way to reduce the strain energy. This may occur by plastic flow through the motion of dislocations, through the creation of surface morphological features which allow strain relaxation, or, particularly when the strain is tensile, through cracking of the film.
Pseudomorphic layers are attractive for at least two reasons. The first is that when an epitaxial layer is grown on a low-dislocation substrate, the pseudomorphic epitaxial layer may also be grown with very low dislocation densities, often with the same dislocation density as the substrate. The second advantage accrues from the ability to tailor the band structure through the large resulting biaxial strains. For example, the strain can be used to break the degeneracy between heavy and light carrier bands and, as a result, obtain higher carrier mobilities.
However, even thick pseudomorphic layers may be insufficient for the fabrication of high-output-power light-emitting devices such as light-emitting diodes (LEDs) and lasers. Such devices are generally sensitive to strain-relieving defects, placing constraints on not only the light-emitting active layer(s) (which are generally one or more strained quantum wells), but also adjoining layers, as defects in adjoining layers may propagate through the active layer(s) even if the active layer(s) remain pseudomorphically strained. Because adjoining layers generally require particular thicknesses and/or compositions to enable, e.g., adequate electrical contact to the device, if these layers are maintained in a pseudomorphic state, the allowable thickness for the active layer(s) (in order to maintain the entire “stack” of layers in a pseudomorphic state) may be diminished, thus decreasing the potential output power of the finished device. Moreover, such layers may require high doping levels to enable low-resistance contacts to the device, and compositions closely lattice-matched with device active layers and/or the underlying substrate may be difficult to dope at high levels. Thus, there is a need for devices having pseudomorphic active layer(s) the thickness of which is not constrained by the strain state of adjoining layers but that remain substantially defect-free (e.g., having a density of defects such as threading dislocations that is approximately equal to the defect level of the underlying substrate), and that are capable of being doped at high levels.